1. Field of the Invention
The present invention concerns techniques for bending glass sheets, and, more specifically, a covering for a solid bending form against which the glass sheet is placed during the bending process.
2. Description of the Prior Art
In the most frequently used bending techniques, a glass sheet, heated in excess of its deformation temperature, is placed, at least once during the bending process, against a bending form comprising a plate made of a heat-resistant sheet metal or ceramic material and embodying, as required, a shape which more or less approaches the final shape which one desires to impart to the glass sheet. This bending form is most often used in combination with a frame having an open, curved profile in its center; accordingly, this form, whether incorporating a flat or curved shape, is currently termed a "solid bending form," in contradistinction to this frame.
Furthermore, positioning the sheet against the solid form, when the latter is curved, may entail a true mechanical pressing of the glass sheet between the solid form and a ring-shaped mold, which is also curved. This pressing operation makes possible the production of windows having complex shapes and/or which are made particularly convex at localized points.
Whether or not a pressing operation occurs during bending, the surface of the solid form coming into contact with the glass sheets is obtained by means of an interposed covering arranged on this form and which must possess well-defined properties.
First, whatever the nature of the covering envisaged, it must be easily attached to the form, so as to make the procedure for changing the interposed covering as short and as simple as possible. To this end, it must, therefore, be able to take on the contours of the form, and, in particular, when a fabric is used, possess, for this purpose, the ability to be well stretched on the form while being relatively flexible and extensible.
Furthermore, once attached to the form, the soft nature of this covering is designed to soften the contact between the glass and the upper form, in which any small imperfections which may subsist in spite of machining are covered over by this sheathing.
In addition, even supposing the bending form to be perfectly smooth, the dust sandwiched between the glass and the form must be taken into consideration. Consequently, the covering must possess a degree of porosity in order to be able to absorb this dust before it threatens to mark the surface of the softened glass.
By virtue of its nature, the texture itself of the covering must not, a fortiori, be capable of leaving imprints on the glass, so as not to impair the optical quality of the windows produced. Therefore, it is desirable that the covering be highly dense and possess a very smooth surface in contact with the glass sheets.
Furthermore, it happens often that enamelling compounds cover the upper surface of the sheet which comes into contact with the upper form.
In fact, these enamelling compounds are applied as batch compounds at the entry to the oven which heats the glass sheets to the bending and/or tempering temperature, and, of course, on the upper surface of the glass sheet, i.e., on the surface which does not come into contact with the conveyor belt, normally of the roller type, which transports the glass through the oven.
Depending on the enamelling compounds used, the enamels, which may not be completely baked when the glass sheet enters the bending station where the bending form is installed, possess a relatively high degree of tackiness. This adherence may cause, on the one hand, damage to the coating, which may be torn away from the form, and it may produce, on the other hand, random delays in the descent of the glass sheets, which must then be compensated for by reduced speed, potentially at the cost of curvature defects.
In addition, said covering must also possess total chemical inertness in relation to the glass sheets, in order that no interfering chemical reaction damage the covering and/or the glass sheets.
According to another crucial requirement, which is added to the preceding ones, the covering must preserve all of its properties intact for the longest possible time at high temperatures. In fact, when attached to the form, the covering must be capable of supporting a temperature greater than 650.degree. C. without deterioration, at least during an interval which is industrially satisfactory, since it is clear that changing the interposed element entails halting production.
A first type of covering used incorporates a fabric made of E glass fibers, i.e., a non-alkaline heat-resistant glass. However, the limiting use temperature of a material of this kind is quite close to the temperature of a glass sheet in a bending chamber; accordingly, this fabric deteriorates rapidly because of oxidation. Moreover, the impression of the weft of the fabric appears on the bent glass sheet, thereby strongly affecting its optical quality.
To solve the problem of deterioration caused by oxidation at high temperatures, the use of silica and/or alumina fibers sintered using a mineral binding agent in order to form a paper has been advocated. The non-woven texture of the paper prevents marking, and the very high degree of resistance to heat of the fibers used protects the covering from any oxidation at the temperature of use. However, it has certain disadvantages.
A paper whose thickness is between approximately 1 to 2 mm must be used if one wishes to stretch it correctly without tearing on the form; but this gives, as a corollary, a stiffness to the paper which is poorly suited to the most complex bending forms, thereby creating a problem arising from the positioning of the interposed element.
In addition, the paper is generally glued against the bending form. If the glue is not applied with the requisite care, small lumps are formed which create blisters whose imprint can be seen on the glass. Moreover, the porosity of the paper allows the glue to shift in spots until it reaches the paper surface; once again, marking of the glass is observed.
Furthermore, when the glass sheets incorporate relatively sticky enamelling compounds, shreds of paper transported by the sheets tend to become detached, thus requiring rapid replacement of the covering.
Use is also conventionally made of a covering incorporating heat-resistant metal fibers assembled together, for example, as knitted fibers, as disclosed in Patent Application FR-A-2 606 398, or as a woven or knitted felt, as proposed by Patent FR-A-2 644 156.
While this type of covering offers other advantages because of the suppleness of its texture, ease of placement on the form, for example, through simple tension produced by clamps, its performance level tends to deteriorate over time at high temperatures. Thus, it may ultimately give a somewhat rough appearance, the fibers tend to thin out and become brittle, and this progressive wear, although occurring less rapidly than that of a fabric made of glass fibers, ends up by marking the glass. This wear results from oxidation, which occurs even when steel fibers termed "stainless" are used under normal bending operation temperature conditions.